1. Field of the Invention
The present invention relates to a device for purifying the exhaust gas of an internal combustion engine.
2. Description of the Related Art
The exhaust gas of an internal combustion engine and, particularly, of a diesel engine, contains particulates comprising carbon as a chief component. Particulates are harmful materials and thus it has been suggested that a particulate filter should be arranged in the exhaust system to trap particulates before they are emitted into the atmosphere. In such a particulate filter, the trapped particulates must be burned and removed to prevent resistance to the exhaust gas from increasing due to the blocked meshes.
In such a regeneration of the particulate filter, if the temperature of the particulates becomes about 600 degrees C., they ignite and burn. However, usually, the temperature of an exhaust gas of a diesel engine is considerably lower than 600 degrees C. and thus a heating means is required to heat the particulate filter itself.
Japanese Examined Patent Publication No. 7-106290 discloses that if one of the platinum group metals and one of the oxides of the alkali earth metals are carried on the filter, the particulates on the filter burn and are removed successively at about 400 degrees C. 400 degrees C. is a typical temperature of the exhaust gas of a diesel engine.
However, when the above-mentioned filter is used, the temperature of the exhaust gas is not always about 400 degrees C. Further, a large amount of particulates can be discharged from the engine. Thus, particulates that cannot be burned and removed each time can deposit on the filter.
In this filter, if a certain amount of particulates deposits on the filter, the ability to burn and remove particulates drops so much that the filter cannot be regenerated by itself. Thus, if such a filter is merely arranged in the exhaust system, the blocking of the filter meshes can occur relative quickly.
On the other hand, when NO2 reacts with the particulates on the particulate filter, the particulates can be burned at a relative low temperature (NO2+Cxe2x86x92NO+CO, NO2+COxe2x86x92NO+CO2, 2NO2+Cxe2x86x922NO+CO2). However, most of NOx included in the exhaust gas is NO and thus NO must be converted to NO2 to make the particulates burn using NO2. Japanese Unexamined Patent Publication No. 8-338229 discloses an oxidation catalytic apparatus arranged upstream particulate filter. The oxidation catalytic apparatus can convert NO to NO2. Further a known NOx absorbent can release the absorbed NO as NO2. Japanese Unexamined Patent Publication No. 8-338229 also discloses that the NOx absorbent is carried on the particulate filter. Thus, NO2 converted by the oxidation catalytic apparatus and NO2 released by the NOx absorbent can burn the particulates on the particulate filter at a relative low temperature. However, in low-engine-load operations, the temperature of the exhaust gas becomes very low, the oxidation catalytic apparatus cannot convert NO to NO2 and the NOx absorbent cannot release NO2. Accordingly, Japanese Unexamined Patent Publication No. 8-338229 discloses that in the low engine load operating area, fuel and secondary air are always supplied into the exhaust system to raise the temperature of the particulate filter by the burned heat thereof. Thus, in Japanese Unexamined Patent Publication No. 8-338229, the fuel consumption rate of the engine deteriorates.
Therefore, an object of the present invention is to provide a device, for purifying the exhaust gas of an internal combustion engine, which can prevent blocking of the particulate filter meshes by the trapped particulates thereon without deterioration of the fuel consumption rate of the engine.
According to the present invention, there is provided a device for purifying the exhaust gas of an internal combustion engine comprising a particulate filter arranged in the exhaust system, on which the trapped particulates are oxidized, wherein the engine can be operated in a first operating mode in which it is given priority to improve the fuel consumption rate thereof and a second operating mode in which it is given priority to regenerate the particulate filter to oxidize the trapped particulates, and one of the first operating mode and the second operating mode is selected to operate the engine at need.